Elevator doors are suspended by hangers that have wheels that ride in or along a track on a lintel attached to an elevator car. The doors are powered open and closed by means of a reversible electric motor that drives a cable attached to the hanger of each door.
An elevator car door opening and closing apparatus of a prior art elevator car 301 is shown in FIG. 11. At one end of a car door header 303, a motor 304 is fixedly installed, and at another end, a driven pulley 309 is installed with a fixed gap. A driving belt 310, with an endless loop, is wound between the motor 304 and the driven pulley 309. At the car door header 303, a car door rail 311 is installed in the longitudinal direction of the door opening 302. Two car doors 312 are respectively hung on the car door rail 311 through car door hangers 313. Each car door hanger 313 has several rollers 314 that make a smooth circular motion along the car door rail 311. The car doors 312 are connected to the driving belt 310 through brackets 315 and 316 attached to the car door hangers 313. In the prior art with this constitution, while the car doors 312 move along the car door rail 311, the car doors 312 are opened and closed through the rotation of the driving belt 310 by the power of the motor 304.
If power is lost, it is desirable for doors to remain in their current position, even if fully or partially open. In fact, some elevator and/or fire codes require that during a power interruption, automatic power operated doors do not move until power is restored and a door open or a closed button is pressed. Contrary to this desire, oftentimes car doors will drift closed at the landing zone due to closing force, from the landing side doors, that overcomes system friction, such as, for example, as a result of closing weights that are typically used to facilitate door closing during normal operation.